Cast iron heat



No Drawing. a plication February 15, 1955 I Serial .No. 488,428

Claims priority, application Francefripiil 9 *6 elm-ms. rei ns-11:8;

The invention relates to improvements in the heat treatment of white cast iron parts which are graphitised by subsequent annealing. It relates more particularly to the process which consists of the following cycle of treatment comprising three phases:

(1) Austenitization followed by interrupted martensitic quenching;

(2) Tempering and nucleation of the graphite at a precise temperature of the order of 450 C., variable according to composition;

(3) Subsequent graphitisation at high temperature (generally between 800 C. and 950 C.).

This process leads to difliculties in the case of the treatment of heavy pieces whose diameters may be as much as and exceed 4 to 7 cm., such as crank-shafts for motor cars, due to the size of the salt baths which are necessary and the difliculty of obtaining an adequate cooling speed to obtain the quasimartensitic structure.

According to the invention it has been foundthat it is possible to obtain good results in the series production of such parts cast in the white state, even in core sand or in shells, without the risk of shrinkage cracks in spite of the size and shape of the parts, if, after the austenitization has taken place at a suitable temperature: between 780 and 880 C. during a period of time sufficient for the homogeneisation of temperature and structural equilibrium to take place (20 minutes to 1 hour 30 minutes), the part is then subjected to a cooling operationin still air until the surface is lowered to a temperature of between 650 C. and 700 C. (or approximately 680 C. to 780 C. at the centre), before being immersed in the hot or cold oil or water quenching tank. This definition of intermediate cooling is characterised by the fact that in those regions of the part which have been cooled to the greatest extent the structure has remained austenitic and is yet able to be hardened by martensitic quenching by immersion in the quenching medium.

It should be noted that by using this delayed quenching with white cast iron parts, it is possible to prevent the formation of shrinkage cracks with perfect reliability.

This process applies particularly to the following cast iron:

Cu Ni 8 P Zr Ti Al I with or without intermediate-cooling, .the: castings a:

2,899,346 e nt brought to 920- C.=-f6r2 hours and are .then cooled'f -J l .1 a The! following :feature's are thus obtained:

flotal absencepf primary cementite-;..

:"Numberbf graphite nodules .iper square millimetre: 2.000to 6.000; Traction eharacteristics: JR= to 4100 'rkgt-lmm a. AZPercent S to -7 f (according-to the French nbrmsrfor 'steels); p" Endurance limit in rotary bending test: E36 iltgt/mmth.

If necessary, after the graphitisation at 920 it'tis possible toscarry ounaspartiar or total: graphitisationof theqaearliteshetween GSOTCJ'andflOOB'G-Jon tolbarlzywut a quality treatment comprising quenching and tempering or to carry out a surface hardening operation.

The process can also be applied to thin parts (i.e. less than 10 mm.) of complicated shape cast in chill moulds with or without pressure. It applies in this case more particularly to the following cast iron composition:

till

0 Mn Si Cu S P Ti Al R=80 to kg./mm. A percent=5 to 7% I claim:

l. A method of heat treating whiteiron castings which comprises subjecting a casting of said white iron to austenization with heat at a temperature of approximately 840 C., cooling the casting in still air sufliciently for the surface temperature of the casting to cool to a temperature in the order of 650 C. to 700 C., quenching the casting to form martensite, subjecting the casting to nucleation tempering with heat at a temperature in the order of 450 C., subjecting the casting to graphitization with heat at a temperature between 800 C. to 950 C. for a period of time in the order of two hours.

2. A method of heat treating white iron castings which comprises subjecting a casting of said white iron to austenization with heat at a temperature of approximately 840 C. for a period of time in the order of 20 minutes to 1 hour and 30 minutes, cooling the casting in still air sufliciently for the surface temperature of the casting to cool to a temperature in the order of 650 C. to 700 C., quenching the casting to form martensite, subjecting the casting to nucleation tempering with heat at a temperature of approximately 450 C., cooling the casting in still air, subjecting the casting to graphitization with heat at a temperature between 800 C. to 950 C. for a period of time in the order of two hours.

3. A method of heat treating white iron castings which comprises, subjecting a casting of said white iron to l austenization of approximately 840 C. for approximately one hour, cooling the casting in still air sufliciently for the surface temperature of the casting to cool to a temperature in the order of 650 C. to 700 C., quenching the casting in oil to form martensite, subjecting the casting to nucleation tempering with heat at a temperature of substantially 440 C. for 4 to 5 hours, subjecting the casting to graphitization with heat at a temperature of about 920 C. for about two hours.

" '4. A'process as defined in claim 3, wherein the cast iron has the composition: 2% C, 1% Mn, 1% Si, 1.35% Cu, 0.75% Ni, 0.05% S, 0.05% P, 0.15% Zr, 0.03% Ti, 0.06% Al, the balancebeing iron.

-5. A method of heat treating white iron castings which comprises,- subjecting a"casting of said white iron to austenization of approximately 840 C. for less than one hour, cooling the-casting in still airsufliciently for the surface temperature "of the casting to' cool to a temperatu'rein the order of 650 C. to 700 C., quenching the casting in oil to form martensite, subjecting the casting to'nucleation tempering with heat at a temperature of substantially-440 C. for 4 to'5 hours, s nbjecting the casting to graphitization with heat at atemperature of about 920 C. for about 2 hours, cooling the casting in still air. 1

" 6.A' 'rocess as defined in claim 3, wherein the cast 4 Cu, 0.05% S, 0.05% P, 0.05% Ti, 0.05% Al, the balance being iron.

References Cited in the file of this patent 5 UNITED STATES PATENTS 2,077,116 Lauenstein-u; Apr. 13, 1937 2,185,894 Hultgren Jan. 2, 1940 2,331,886 Boegehold Oct. 19, 1943 2,501,059 Kluijtmans Mar. 21, 1950 Hune Sept. 8, 1953 OTHER REFERENCES Materials and Methods, vol. 32, issue 6, December 1950, pages 50-53. 15 Rehder: The Iron Age, Ian. 17, 1952, pages 89-93.

iron has the composition: 2%'C, 1% Mn, 1.5% Si, 1.35% V 

1. A METHOD OF HEAT TREATING WHITE ION CASTING WHICH COMPRISES SUBJECTING A CASTING OF SAID WHITE ION TO AUSTENIZATION WITH HEAT AT A TEMPERATURE OF APPROXIMATELY 840*C., COOLING THE CASTING IN STILL AIR SUFFICEINTLY FOR THE SURFACE TEMPERATURE OF THE CASTING TO COOL TO A TEMPERATUARE IN THE ORDER OF 650*C. TO 700*C., QUENCHING THE CASTING TO FORM MARTENSITE, SUBJECTING THE CASTING TO NUCLEATION TEMPERATURE WITH HEAT AT A TEMPERATURE IN THE ORDER OF 450*C., SUBJECTING THE CASTING TO GRAPHITIZATION WITH HEAT AT A TEMPERATURE BETWEEN 800*C. TO 950*C. FOR A PERIOD OF TIME IN THE ORDER OF TWO HOURS. 